Investigation on the Influence Mechanism of Gas Desorption on Coal Chemical Structure and Oxidation activity

To reveal the variation mechanisms of coal seam gas desorption capacity, micro-macro properties, and gas generation laws during temperature-programmed oxidation under deep high-temperature and high-pressure conditions, and address the insufficient research on temperature-pressure coupling effects in current deep coal seam disaster prevention, this study took non-caking coal (BN) from a coal mine in Shaanxi Province and lean coal (SM) from a mine in Liaoning as research objects. A series of experiments, including ultimate analysis, adsorption-desorption, scanning electron microscopy (SEM), thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) coupled system, and temperature-programmed oxidation, were conducted to systematically investigate the regulatory mechanism of temperature-pressure coupling on coal gas desorption, as well as the influence of gas desorption on coal structure and oxidation activity. The results showed that temperature-pressure coupling significantly regulated gas desorption, with the inhibitory effect of temperature being stronger than the promoting effect of pressure. The SM coal (with higher coalification degree) exhibited an overall higher desorption capacity than BN coal. SEM experiments for the first time in this study, clarified the differential pore characteristics of coal samples before and after desorption: raw coal was dominated by gas pores, cell cavity pores, and mold pores, while desorbed coal was mainly composed of brecciated pores and friction pores. Additionally, gas desorption altered the chemical structure and oxidation activity of coal. TG-FTIR results indicated that after desorption, the absorption peak intensities of oxygen-containing functional groups (O-H, C = O) and aliphatic hydrocarbon structures (-CH3, -CH2-) in coal samples significantly decreased, and the termination temperature of the oxidation weight-gain stage was lowered. The temperature-programmed oxidation experiments innovatively proposed a dual-parameter identification criterion of “characteristic gas temperature threshold + critical oxygen concentration”: the critical generation temperatures of CO, C2H4, and C3H6 were 49°C, 141°C, and 141°C (141°C marks the transition of oxidation to the accelerated stage), respectively. Moreover, the concentration of characteristic gases in desorbed coal was significantly lower than that in raw coal, and the CO concentration under 20.9% oxygen (air) was an order of magnitude higher than that under 7% oxygen. These findings clarify the intrinsic relationships among coalification degree, desorption characteristics, microstructures, and oxidation laws, define the differential prevention and control strategies for the “three zones” in goafs, and provide quantitative support and a replicable engineering paradigm for disaster prevention and control in deep coal seams of high-gas and spontaneous combustion-prone mines.